I have a HD500 and have been observing an issue that occurs pretty frequently: If I leave the unit on for a longer time (above 1 hour, but times vary), the tone I hear changes, without having changed any of the settings in the patches. It gets more trebly and harsh, almost like changing from studio direct to combo amp, but more subtle. I had been noticing this using with my headphones (Audio Technica M50) and thought maybe the headphones were to blame, but noticed the same effect happening connected to my DT50 (with L6link) a few times.
Any Idea what might be happening? anybody else ever had that problem?